The present invention relates generally to a method of and apparat us for reducing ghost artifact in image data. The invention has particular application in those Magnetic Resonance Imaging (MRI) techniques which are prone to coherent ghost artifact.
In conventional Two Dimensional Fourier Transform (2DFT) MRI techniques, one xe2x80x9clinexe2x80x9dof data in sample space (also known as k-space) is acquired for each application of a radio frequency (rf) pulse. The resulting echo signals are sampled at various points during the evolution of the magnetic gradients in the xe2x80x9cread directionxe2x80x9dto obtain each line. Incremental increases in the magnetic gradient are applied in the xe2x80x9cphase encode directionxe2x80x9d in order to read successive lines through the sample. A two dimensional Fourier transformation is applied to the sampled data to obtain the image data.
Under certain circumstances there may be a periodic variation in the sampled data, leading to the appearance of a ghost artifact in the image data. The periodic variation may be in any of the parameters of the sampled data, for example the amplitude or the time of sampling. Any phenomenon that causes a periodic variation in the sampled data may give rise to a ghost artifact. Typical examples are periodic movements in the sample, such as may occur when imaging the heart or lungs, or periodic variations in the operating conditions of the imaging apparatus, which may be due to internal or external influences. In the case of the periodic movement of the heart, a coherent ghost artifact will appear if the heart rate is related to the sampling rate.
In certain MRI techniques, there may be a periodic variation in the sampled data due to the manner in which the data are acquired. Examples of such techniques are Echo Planar Imaging (EPI), Segmented EPI, and Echo Volumar Imaging (EVI).
Echo Planar Imaging differs from 2DFT in that an entire image is acquired from a single rf excitation pulse. To acquire the image, increments of magnetic gradient in the phase encode direction are applied, whilst switching the magnetic field in the read direction between positive and negative. The echo signals are sampled at various points during the evolution of the magnetic gradients to obtain sampled echo data. A two dimensional Fourier transformation is then applied to the sampled data to obtain the image data.
EPI intrinsically involves a periodic variation in the sampled data as a result of the alternate switching of the magnetic field in the read direction between positive and negative. This switching of the magnetic field results in alternate lines in the sampled data requiring time reversal prior to Fourier transformation. Any misalignment between the time reversed lines will result in a coherent ghost appearing in the image, overlapping with the real image.
Segmented EPI works by applying a number of excitation pulses, and acquiring part of the data, known as a segment, following each pulse. Changes in the operating conditions between the different segments, together with the switching of the magnetic field within each segment, may give rise to periodic variations in the sampled data with a periodicity of twice the number of segments. This will give rise to multiple ghost artifacts in the image domain.
For the purpose of simplicity much of the description henceforth will be directed towards EPI, which is highly prone to a single coherent ghost artifact at +Np/2 in resulting images, where Np is the number of points in the phase encode direction. However, the present invention is applicable to any imaging technique in which a coherent or pseudo-coherent ghost artifact occurs, regardless of the origin of the ghost artifact.
Various techniques have been used to reduce or cancel ghost artifact in EPI. Perhaps the simplest method is a manual technique, in which relative time shifts in the sampling points between time reversed lines are adjusted manually, until the ghost disappears. This has the disadvantage that it requires the intervention of a skilled operator. It also requires that the system has real time data acquisition, reconstruction and image display facilities.
An alternative technique for ghost elimination has been proposed, in which a calibrating scan is first done to determine the time shift between the time reversed lines of data. This time shift is then used to correct the errors in subsequent imaging scans. This has the disadvantage of being complex and of increasing the time required for imaging. Furthermore, there may be changes in the time shift between the calibrating scan and the imaging scan, leading to a reduction in the efficiency of the ghost cancelling.
Methods employing additional, redundant, reference scan lines within the imaging sequence have been proposed (Jesmanowicz et al SMRM abstract, 1993, p 1239 and EP 0644 437A, Philips Electronics NV, 1995). As these require extra data sampling these methods prolong the data acquisition and hence the imaging times, which is critical in high speed imaging techniques.
A post processing method of reducing ghost artifact has been proposed by Bruder et al in Magnetic Resonance in Medicine, vol. 23, pp 311-323, (1992). Bruder et al proposed automatic adjusting of the data in the image domain until the ghost substantially disappears. This technique relies on the genuine image and the ghost image being spatially separated, that is, not overlapping in the imaging field. The technique therefore cannot be used if there is a genuine image across the whole of the imaging field. In EPI this is a significant disadvantage, since the size of the imaging field is limited by the ability of the imaging apparatus to generate large and rapidly varying magnetic fields. Consequently, it is highly undesirable to reduce the area allocated to the genuine image even further, by dedicating an area for use in ghost artifact reduction.
A similar technique to that of Bruder et al has been proposed by Buonocore et al in Magnetic Resonance in Medicine, vol. 38, pp 89-100, (1997). The technique of Buonocore et al also relies on analysing the ghost image in isolation and therefore can only be used when there is an area of no overlap between the genuine image and the ghost image. This technique therefore suffers from the same disadvantages as that of Bruder et al.
If employed with imaging techniques such as Segmented EPI, where multiple ghost artifacts occur, the techniques of Bruder et al and Buonocore et al would require the collection of correspondingly more lines dedicated to ghost removal and devoid of genuine image rendering them even less practical.
It is an aim of the present invention to provide a method of and apparatus for reducing ghost artifact that overcomes or reduces the problems of the prior art.
In one aspect, the invention provides an apparatus for reducing ghost artifact in image data, the apparatus being for use with an imaging apparatus which produces sampled raw image data that may experience a periodic variation giving rise to said ghost artifact, the apparatus comprising image reconstruction means for converting the sampled raw image data into the image data to reconstruct an image, characterized by means for analysing the sampled raw image data alone to determine a correction to reduce the ghost artifact, without requiring additional sampled data beyond that required by said image reconstruction means.
Analysing the sampled data (rather than the image data) to determine the correction can afford the advantage that a correction to reduce ghost artifact may be calculated without requiring extra data to be acquired. At the same time, the present invention can be readily implemented with existing magnetic resonance imaging techniques as it does not necessitate any changes in data sampling procedures. The need for operator interaction in reducing ghost artifact may be avoided. Analysing the sampled data, rather than adjusting the operating conditions, gives the advantage that ghost artifact may be reduced regardless of the origin of the artifact.
It has been realised pursuant to the present invention that coherent ghost artifacts may result from an offset of actual sampling points from desired sampling points, that is, those required by the Fourier transform for correct, artifact free, image reconstruction. Therefore the analysing means may comprise means for determining an offset of an actual sampling point in the sampled data from a desired sampling point to thereby determine the correction.
The determining means may be adapted to determine the offset of an actual sampling point from the point of maximum signal in the sampled data, since this can afford an efficient and convenient way of determining the offset. The point of maximum signal is typically where all rf components are coherent. For simplicity, the determining means may be adapted to perform a fitting procedure on the sampled data to estimate the point of maximum signal.
The apparatus may further comprise means for separating the sampled data into two or more data sets and the analysing means may be adapted to determine a correction for at least one such data set; in this case the correction may be an offset of an actual sampling point from a desired sampling point in that data set. Each data set may contain data that were acquired at corresponding points in different cycles of the periodic variation.
For example, in EPI, there is a periodic variation in the sampled data due to the alternate switching of the magnetic field in the read direction between positive and negative. The sampled data may therefore be separated into two data sets, one containing the data acquired with the positive field, and the other containing the data acquired with the negative field. In Segmented EPI, the sampled data may be separated into a number of data sets equivalent to twice the number of segments, and each containing the data acquired with a positive or negative field in one segment. In general, the number of data sets will correspond to the number of images (genuine and ghosts).
The analysing means may be adapted to determine a correction for one data set relative to another. This can allow one data set to be corrected relative to another, rather than having to correct each data set individually; hence the amount of processing can be reduced.
It has been realised pursuant to the present invention that a correction may advantageously be applied to the data sets after transformation, as this can allow a simple phase correction to be applied to the transformed data. The apparatus may therefore further comprise means for separately transforming the data in each data set, and, if so, means may be provided for converting the correction to a correction to be applied to transformed data, and for applying the converted correction to transformed data.
The analysing means may be adapted to determine a correction for substantially all of the data in each data set and the correction applying means may be adapted to apply the converted correction to substantially all of the transformed data in one or more of the data sets. This can reduce the amount of processing, since the same correction is applied to all of the data in a data set.
In a first preferred embodiment, the transforming means may be adapted to perform a first one dimensional Fourier transformation, the correction applying means may be adapted to apply the converted correction to the transformed data in one or more of the data sets, and the apparatus may further comprise means for performing a second one dimensional Fourier transformation on the transformed data. The second one dimensional Fourier transformation may be orthogonal to the first.
In a second preferred embodiment, the transforming means may be adapted to perform a two dimensional Fourier transformation. In this case, the correction is applied to substantially all of the data in one or more of the data sets following a two dimensional Fourier transformation. A two dimensional Fourier transformation can be more efficient than two orthogonal one dimensional Fourier transformations.
In a third preferred embodiment, instead of applying a correction to all of the data in a data set, the analysing means may be adapted to determine a correction for a portion of the data in each data set and the correction applying means may be adapted to apply the converted correction to a portion of the transformed data in one or more of the data sets. Each data set may thus be divided into a plurality of portions and each portion may correspond to one line in each data set, or to a group of lines, or some other subset. In this case, a correction may be calculated individually for each portion and a converted correction applied to each of the corresponding portions. This may be advantageous when the correction that needs to be made varies within the data set. This situation may arise due to imperfect gradients and field inhomogeneities.
The transforming means may be adapted to perform a first one dimensional Fourier transformation, the correction applying means may be adapted to apply the converted correction to a portion of the transformed data in one or more of the data sets, and the transforming means may be further adapted to perform a second one dimensional Fourier transformation on the transformed data. The second one dimensional Fourier transformation may be orthogonal to the first.
The analysing means may be adapted to determine a correction for one portion of the data in a data set from the correction for another portion of the data in a data set. This can allow corrections to be determined for portions having data values with low Signal to Noise Ratios (SNR) which would otherwise give poor estimates of the corrections.
In any of the first three preferred embodiments, the apparatus may comprise means for transforming the sampled data, means for converting the correction to a correction to be applied to the transformed data, and means for applying the converted correction to the transformed data.
In a fourth preferred embodiment, the apparatus may further comprise means for applying the correction to the sampled data before conversion to image data. This may be preferred, for example where some other processing is to be carried out on the data prior to transformation.
In another aspect, the invention provides an imaging apparatus which is adapted to sample data, comprising the apparatus as aforesaid. The imaging apparatus may suitably include means for exciting nuclear magnetic resonance and means for detecting the resonance response signals. Further, the imaging apparatus may be adapted to sample data in lines, some of which are time reversed with respect to others. The imaging apparatus may for example be adapted to perform Echo Planar Imaging or Segmented Echo Planar Imaging. Alternatively, the imaging apparatus may be adapted for imaging three or more magnetic resonance dimensions such as in Echo Volumar Imaging or in the simultaneous acquisition of two spatial imaging dimensions and one chemical shift dimension.
In another closely related aspect, there is provided a method of reducing ghost artifact in image data, the method being for use with an image reconstruction technique in which sampled raw image data are produced and converted into the image data, the method characterised by analysing the sampled raw image data alone to determine a correction to reduce the ghost artifact without requiring additional sampled data beyond that required for image reconstruction.
The step of analysing the sampled data may comprise determining an offset of an actual sampling point in the sampled data from a desired sampling point to thereby determine the correction. The step of determining the offset may comprise determining the offset of an actual sampling point from the point of maximum signal in the sampled data. The step of determining the offset may comprise performing a fitting procedure on the sampled data to estimate the point of maximum signal.
The method may further comprise separating the sampled data into two or more data sets and determining a correction for at least one such data set. The step of analysing may comprise determining a correction for one data set relative to another.
The method may further comprise separately transforming the data in each data set. The method may further comprise converting the correction to a correction to be applied to transformed data, and applying the converted correction to transformed data.
The step of analysing may comprise determining a correction for substantially all of the data in each data set and the step of applying the converted correction may comprise applying the converted correction to substantially all of the transformed data in one or more of the data sets.
The step of transforming may comprise performing a first one dimensional Fourier transformation, the step of applying the converted correction may comprise applying the converted correction to the transformed data in one or more of the data sets, and the method may further comprise performing a second one dimensional Fourier transformation, which may be an orthogonal one dimensional Fourier transformation, on the transformed data.
Alternatively, the step of transforming may comprise performing a two dimensional Fourier transformation.
The step of analysing may comprise determining a correction for a portion of the data in each data set and the step of applying the converted correction may comprise applying the converted correction to a portion of the transformed data in one or more of the data sets.
The step of transforming may comprise performing a first one dimensional Fourier transformation, the step of applying the converted correction may comprise applying the converted correction to a portion of the transformed data in one or more of the data sets, and the step of transforming may comprise performing a second one dimensional Fourier transformation, which may be an orthogonal one dimensional Fourier transformation, on the transformed data.
The step of analysing may comprise determining a correction for one portion of the data in a data set from the correction for another portion of the data in a data set.
The method may comprise transforming the data, converting a correction to a correction to be applied to the transformed data, and applying the converted correction to the transformed data.
The method may further comprise applying the correction to the sampled data before conversion to image data.
In a further aspect, the invention provides an imaging method in which data is sampled, comprising the method as aforesaid. The imaging method may sample data in lines, some of which are time reversed with respect to others. For example, the imaging technique may be Echo Planar Imaging or Segmented Echo Planar Imaging. Alternatively, the imaging technique may be for imaging three or more magnetic resonance dimensions such as in Echo Volumar Imaging or in the simultaneous acquisition of two spatial imaging dimensions and one chemical shift dimension.
In yet a further related aspect, the invention provides a computer disc containing software for carrying out the method described above.